JPH05192631A - Electrostatic coating method - Google Patents

Abstract

PURPOSE: To prevent the degradation in the quality of coating by a conductive film by forming the continuous conductive film as a continuous solid conductive film formed on the rear surface of a panel and forming this conductive film in a manner as to have the resistivity smaller a specific value. CONSTITUTION: This electrostatic coating method is a method for electrostatic coating the outside surface of a nonconductive article by a coating device and has a stage for forming the continuous conductive film on the inside surface of the article and a stage for electrostatic adhering coating material on the outside surface of the article. The outside surface of the article is formed as the front surface of the nonconductive panel and the inside surface of the article is formed as the rear surface of the panel. The continuous conductive film is formed as the continuous solid conductive film to be formed on the rear surface of the panel. The conductive film is so formed as to have the resisitivity smaller than 10<9> ohm per square meter. The coating is thus executed electrostatically in the state of applying the conductive coating on the surface on the side opposite to the surface to be coated.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to electrostatic coating or electrostatic coating technology, and more particularly to, for example, US Pat. No. 3,741,7.
No. 93, a step of forming a continuous conductive film on an inner surface of an article, a step of generating an electrostatic field between the conductive film and a coating device, and an outer surface of the article. Electrostatically depositing a coating material on the outer surface of the non-conductive article (by a coating device).

[0002]

2. Description of the Related Art Electrostatic spray coating of a plastic material is a spray used to sufficiently cover plastic parts because the transfer efficiency is high when the plastic material is coated using electrostatic adhesion. Commercial (economic) because the amount of paint is small and the amount of emission is small.
I am interested in. In the electrostatic coating method, it is necessary to generate an electrostatic field suitable for adhering charged paint particles around the article being coated. If the article itself is conductive, the article is electrically grounded and the article itself is used as an electrode for the electrostatic field. If the article is non-conductive, a high molecular weight (primer) or low molecular weight conductive coating material (coating) is applied to the surface to be coated, and the conductivity of the surface is sufficient to function as an electrode. Increase.

In the case of conductive primers, the primer may need to be used for other purposes in addition to its sole function of providing a conductive surface. The primer can provide a good surface for painting, prevent penetration of solvents into the plastic surface during painting, and have various other functions. In most cases, these other properties are sacrificed to render the primer conductive. Therefore, the need to provide conductivity limits the preparation for suitable primers.

The low molecular weight coating material is an organic salt or a mixture of organic and ionic materials (the ionic family being positioned to generate a conductive surface) which has the property of being adsorbable on the surface of plastic materials. Cosmetic problems (the coating material is visible through the color coating and sometimes alters the color, uniformity and definition of the coating image and other visual features) and the coating material and plastic material after the coating material is cured. Interfering with each other's) adhesive problems. After a long time, the covering film may be separated from the plastic material due to poor adhesion.

In order to avoid the use of such conductive coating materials on the surface to be painted, the back side of the plastic panel (grounded) is provided to provide terminations (terminals) for the electrostatic paint application field. ) A technique for disposing a metal support plate has already been proposed. While such techniques are useful when the appearance and uniformity of the coating are not a significant issue, such support plates may provide intimate contact with the panel as a suitable field electrode or adequate fit to the article. (As a result, a non-uniform distribution of the paint is generated), it is necessary to secure a large number of metal plates with special shapes for each panel shape, which leads to high cost, and it is necessary to attach a metal plate to the panel or remove it from the panel. You have to remove the board.

Further, as disclosed in US Pat. No. 3,741,793, the back surface of the article is covered with water or other conductive liquid, and this liquid is grounded to cause electrostatic discharge on the front surface of the article. A technique of acting as an electrode for attachment has also been proposed. In particular, the above-mentioned U.S. Pat. No. 3,741,793 discloses a technique of introducing steam into the inside of a hollow article and sufficiently cooling the article to condense the steam on the inner wall of the article.

[0007]

However, the above-mentioned US patent specification does not disclose how to apply the above-mentioned technique to an open article such as a panel, and the strict appearance specification is satisfied. There is also no disclosure of whether or not a coating having sufficient conductivity and continuity to provide a high quality coating to be obtained is obtained. In any case, the use of such liquid electrodes in articles can be expected, but their application is limited to specialized painting operations such as painting bottles. In environments such as automotive part painting, it is customary to mount the part on a carrier, wash the part, and then paint the part.
When the liquid conductive coating material is applied to the part, the coating material is washed off before the painting process. In addition, the parts are exposed to high temperatures between application of the liquid conductive coating material and painting, causing a considerable time delay so that the liquid coating material dissipates before it can perform its function.

Therefore, it is an object of the present invention to provide a method of electrostatically coating the surface after applying a coating material which is a solid conductive film having sufficient durability to withstand the painting operation.

[0009]

The method of electrostatically coating the outer surface of a non-conductive article according to the present invention is characterized in that the outer surface of the article is the front surface of the non-conductive panel, and the article is the inner surface and the rear surface of the panel, continuous conductive film by a continuous solid conductive film formed on the back surface of the panel, the conductive film is one square per 10 ⁹
It has less resistance than ohms.

The present invention is practiced by a method of electrostatically coating the front surface of a non-conductive panel with a coating apparatus. This method comprises the steps of forming a continuous solid conductive film (which has a resistance of less than 10 ⁹ ohms per square) on the back surface of the panel, and a static conductive film between the solid conductive film and the coating device. It has a step of generating an electric field and a step of electrostatically attaching a coating material to the front surface of the panel.

[0011]

EXAMPLES In the manufacture of automobiles, plastic panels for automobile body parts include reaction / injection / molding materials (RIM), thermoplastic olefins (TPO), thermoplastic urethanes (TPU) and sheet molding compounds (S).
MC). Molded panels have different surface properties and therefore cannot all be treated in exactly the same way. Some embodiments of the invention for use in plastic panels are described, some of which are more suitable for other panel materials besides plastic.

The method of providing a solid conductive coating material on the back surface of a panel has already been described for applying a film or coating to the front surface of the panel to provide conductivity for electrostatic coating or for other purposes. It is used. Therefore, coating techniques and molding techniques themselves are known and will not be described in detail.

The purpose of applying a conductive coating material to the front surface of a panel for electrostatic coating has conventionally been twofold. That is, to provide a grounded surface for generating an electrostatic field between the panel and the coating device, and to dissipate the charge associated with paint droplets deposited on the surface. The premise of the present invention is that the function of providing a grounded surface for electrostatic fields is best performed by a conductive surface on the back side of the panel, and surface charge dissipation is performed well by a wet paint applied to the front surface. Is to be done. Although the solution in the paint has sufficient conductivity for this purpose, the paint is generally considered non-conductive after curing.

One method of applying a conductive coating to the back surface of a plastic panel is to spray a salt solution onto the back surface and then dry the back surface. Solutions of this kind are known for application to the front surface of the part to be coated. In particular,
Test panels made of TPU, SMC, TPO and RIM have a conductive quaternary ammonium salt solution concentration of 1.9 mol% per liter (eg Cyastat S from American Cyanamid USA).
The solution of N) was sprayed on. This solution had a 20:80 water to isopropanol ratio for the TOP panel and 100% water for the other material panels.
The salt solution was applied to the side opposite the side to be painted. After the spraying was completed, it was left to dry.

The paint transfer efficiency of the treated panel was measured by applying a coating to the treated panel and a metal panel of the same size. The coating was performed using a commercially available rotary bell-type electrostatic spray gun using a commercially available automotive paint. The weight gain (increase) of each panel was measured and the weight gain of each test panel was compared to that of the associated metal panel to determine transfer efficiency. The transfer efficiency for the panels made of SMC, TPO, TPU, and RIM is 63 each.
%, 103%, 90% and 72%. The same test was carried out on untreated panels and it was found that the transfer efficiency was between 14% and 64%, which showed that the surface treatment of the rear surface significantly improved the paint adhesion. The difference between the treated panels depends on the difference in the surface characteristics of the panels. Panels with low transfer efficiencies had greater surface porosity and the salt solution that dried to a solid film did not form a continuous film, thus causing greater resistance to the solid film. A smooth surface, such as a TPO panel, produced a lower resistance value because a continuous film was obtained and had the best paint transfer efficiency. Tests show 1 for 1 square for proper paint adhesion
It turns out that a resistance of ⁰⁹ ohms is needed.

The above-mentioned sprayed salt coating material can be washed off, which is advantageous if it is desired to remove the conductive coating material, but the coating material is fragile and can be washed off before painting by washing the panel. It is disadvantageous if it is done. Other types of coating materials or films can be used if a permanent conductive layer is preferred.

One example of a permanent coating is a conductive primer that is sprayed onto the backside of the panel. Since the usual properties of the primer, such as promoting adhesion, are not relevant when applied to the backside of the panel, the primer can be prepared for optimal conductive properties. For example, it may contain a large amount of carbon black. The primer is sprayed onto the back of the panel and allowed to dry. The panel is then mounted on a grounded conveyor hanger and the front surface of the panel is electrostatically painted.

Instead of providing a conductive coating on the panel after molding, such a coating may be formed in the mold as part of the panel. A conductive primer or other conductive coating material is sprayed onto the mold surface to form the back surface of the panel, forming a conductive layer on the mold surface as the back surface of the panel when the panel is formed in the mold. Good. Instead of spraying the liquid into the mold, insert a sheet of electrically conductive material such as aluminum foil or metallized plastic film into the mold,
It may be molded on the surface of the panel. Such examples of molding to provide a surface layer for other purposes are well known in the molding industry.

Another method of forming a conductive back surface on a panel is the in-mold coating method. In this method, the panel is formed in a mold, partially cured, the mold pressure is released, a conductive coating material is injected into the mold to cover the back surface of the panel, and curing is completed to remove the conductive material. Include as part of the panel.

Cleaning, regardless of how the conductive layer is applied to the back surface of the molded panel
The conductive coating material remains on the panel during the process or other panel preparation steps. The conductive layer is then grounded and an electrostatic field between the coating equipment and the grounded rear surface of the plastic panel (the strength of this field is used to help deposit charged paint particles on the front surface of the panel). The paint is electrostatically sprayed onto the front surface of the panel by a device that creates a high voltage in the coating equipment to produce Typically, a portion of the paint wraps around the panel resulting in a continuous coating of paint over the edges of the conductive layer on the front surface to the back surface of the panel. This provides a conductive path in the sprayed paint, allowing the charge on the front surface of the panel to flow to the grounded conductive layer on the rear surface of the panel.

The finished product has an excellent painted appearance and can be mainly applied to automobile body panels, appliances and other high quality articles. The method of the present invention, which utilizes the grounded surface, provides excellent quality in terms of coating thickness uniformity, image sharpness, and other standard coating quality and subjective observation results. ..

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ala Abdel-Azim El Mourge 48313 Sterling Heights, Michigan, USA 15022 Annapolis

Claims (9)

[Claims] 1. A method of electrostatically coating an outer surface of a non-conductive article with a coating device, the method comprising forming a continuous conductive film on the inner surface of the article, the conductive film and the coating device. In the electrostatic coating method having a step of generating an electrostatic field between and, electrostatically attaching a coating material to the outer surface of the article, the outer surface of the article as the front surface of the non-conductive panel, The inner surface of the article is the back surface of the panel, and the continuous conductive film is a continuous solid conductive film formed on the back surface of the panel. The conductive film has a resistance of less than 10 ⁹ ohms per square. An electrostatic coating method, characterized by having an index. 2. The step of forming a solid conductive film on the back surface of the panel comprises the steps of spraying a solution containing a conductive material on the back surface of the panel, and drying the solution to form the solid conductive film on the back surface. And a step of forming a conductive film. 3. The electrostatic coating method according to claim 2, wherein the solution containing the conductive material is an aqueous solution of a quaternary ammonium salt. 4. The step of forming a solid conductive film on the back surface of the panel comprises the steps of spraying a conductive paint on the back surface of the panel, and drying the paint to form the solid conductive film on the back surface. The method of forming an electrostatic coating according to claim 1, further comprising a step of forming. 5. The panel is formed by molding; the step of forming a solid conductive film on the back surface of the panel is performed before the panel is molded so that the panel is directly coated by the molding. The electrostatic coating method according to claim 1, further comprising the step of applying a conductive film in the mold. 6. The electrostatic coating method according to claim 5, wherein the conductive film is applied by spraying a coating material of a conductive primer on the surface of the mold. 7. The electrostatic coating method according to claim 5, wherein the conductive film is applied by inserting a sheet of metal foil into the mold. 8. The electrostatic coating method according to claim 5, wherein the conductive film is applied by inserting a metallized plastic film into the mold. 9. The step of forming the panel by molding; the step of forming a solid conductive film on the back surface of the panel comprises the steps of partially curing the panel within the mold and the partially cured panel. Spraying a conductive material into the mold to cover the back surface of
The electrostatic coating method according to claim 1, further comprising the step of completely curing the panel in the mold.